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vertex – ZFC
vertex
Geometry: In a plane figure, a point which is a common end-point for two or more lines or curves.
 Graph Theory: One of two kinds of entities in a graph.
Cf. edge.
vertex set
The set of vertices of some graph. For a graph G, the vertex set of G is denoted by V(G), or, if there is no ambiguity as to the graph in question, simply by V.
Von Neumann Heirarchy
ARTICLE
A construction in set theory giving rise to the class of ordinals. One begins with the empty set, and successor sets are obtained by forming the union of the current set and the set containing the current set; at limit stages, take unions. See the article for a detailed exposition.
weakly connected digraph
A directed graph every two vertices of which are connected by a semipath. Also called weak for short. A directed graph is weak if and only if it has a spanning semiwalk.
Cf. unilaterally connected digraph, strongly connected digraph, connected graph.
well-founded
In set theory, a collection is well-founded if every subcollection has a least member under the membership relation. For example, the set of natural numbers is well-founded. In ZFC, the foundation axiom asserts this property of all sets. A set which is not well-founded is sometimes called a hyperset.
well-ordered
A set S with a linear order is called well-ordered if every non-empty subset T of S has a least element under the ordering relation.
Cf. well-ordering principle.
well-ordering principle
The assertion that every set can be well-ordered. Equivalent to the Axiom of Choice.
whole number
An element of the set consisting of the number 0 together with the counting numbers, 1, 2, 3, etc.; i.e., the set N of natural numbers with 0 included. Sometimes the term “whole” is meant to refer to negative integers also; the intended meaning should be clear from the context.
Zeno’s Paradox of the Arrow
One of the famous paradoxes of Zeno of Elea. Consider an arrow in flight. At each moment of time the arrow may be considered to occupy a specific region in space. This poses the following problem: if the arrow is in a particular point of space in any given moment, then how is it moving? And if it is not moving, how does it get from point to point in the passage of time?
Zermelo Fraenkel set theory
The standard set theory in which most mathematics is formalized. Its axioms include the pairing axiom, the power set axiom, the axiom of infinity, the axiom of extensionality, the axiom of replacement, the separation axiom, the union axiom, and the foundation axiom. Abbreviated ZF. When the axiom of choice is assumed, this theory is abbreviated ZFC.
ZF
See Zermelo Fraenkel set theory.
ZFC
Zermelo Fraenkel set theory with the axiom of choice.
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